Harry Gibbons, PhD Gene Welch, PhD. April 25 th, Tetra Tech, Inc.

Transcription

1 Harry Gibbons, PhD Gene Welch, PhD April 25 th, 2013 Tetra Tech, Inc. 1

2 Reducing P-Alone-is the Key to Managing Eutrophication in Lakes Especially Cyanobacteria Some believe N should be reduced too N reduction not recommended too, even though limiting in short term, because: Bottle/mesocosom experiments stop short and do not allow N-fixation to build up the N supply observed in whole-lake, long-term studies----only reliable evidence for policies to reduce eutrophication. Reduction of N too could continue to favor N fixers and is several times the cost of P reduction alone. No cases that N reduction alone will reduce trophic state, but many successful cases of P reduction alone, Schindler lists 35-Jeppesen et al., 2005 (those with N reduction too responded similarly to those with P alone). Schindler, D.W The dilemma of controlling cultural eutrophication. Proc.Royal Soc.B. Over 170 alum treatments to lakes, which demonstrate P reduction alone is sufficient to demonstrate P management is key. 2

3 Once a Lake is Pushed beyond its Eutrophic State by Watershed Abuses: In-Lake Activities Have to be the Center of the Game Plan Primary production and related water quality is a direct function of phosphorus availability Relative when and how much P is available within the lake For many lakes with current or past excess external P loading it is not the orginal source of phosphorus that is important: It is quantity and timing of phosphorus availability within the lake that is important! 3

4 In-Lake Quantity and Timing of Phosphorus Availability Magnitude of internal P loading Relative to external sources, often is largest contributor Especially in summer This internal loading drives the cyanobacteria production Can continue for decades after external abuses are reduced To maintain beneficial uses of lakes in-lake activities are needed Often inactivation of internally loaded phosphorus is essential to success, regardless of external controls 4

5 In-Lake Quantity and Timing of Phosphorus Availability cont Eutrophic lakes usually have long water residence times and significant internal P loading during summer Fourteen lakes (9 unstratified and 5 stratified) in Western WA had summer flushing rates averaging 20± 9% of annual and internal P loading averaging 69 ± 19% of summer total loading (Welch and Jacoby, 2001) That means during summer when algal blooms occurred, internal supplies most of the P to algae because external P loading was low That is especially true in Western WA where winters are rainy and summers are dry 5

6 Relationship between mean summer (May- September) TP and chlorophyll a in nine unstratified lake basins Summer chl was strongly related to summer TP in the 9 unstratified lakes/ basins where summer TP loading was mostly internal and immediately available to algae 6

7 Legacy Phosphorus Example: Lake Ketchum Original source of P from former dairy farm that drains to Lake Ketchum inlet Over time P stored in lake sediments Now the major source of pollution to the lake; SRR = 42 mg/m 2 -day Since 1996 several improvements made to runoff reducing inlet TP concentrations Inlet TP dropped from ~ 1,500 µg/l in to 646 µg/l in Epilimnetic TP also declining; 181 µg/l summer average in 2011 (long term summer average is 277 µg/l) Hypolimnetic TP increasing; 2,667 µg/l 2011 summer average compared to long term average of 1,746 µg/l 7

8 Legacy Phosphorus Example: Lake Ketchum Internal loading accounts for 73% of total annual P load to Lake Ketchum; mostly during summer Main cause of HABs in Lake Ketchum Treatment Alternatives Must control internal P loading in order to control HABs and have any improvement in lake water quality Restoration TP target = 40 µg/l Excerpt from Lake Ketchum Algae Control Plan, Snohomish County & Tetra Tech, June

9 Internal loading even greater % in many shallow hypereutrophic lakes Lake Area (ha) Mean Depth (m) TP 1 µg/l % Internal Load 1 Upper Klamath Lake, OR 26, , 59 2 Arresø, DK 4, , 71 2 Vallentuna, SK , 87 2 Søbygaard, DK , 55 2 GLSM, OH 5, , Summer (4 months) 2 Annual 9

10 Success of alum treatments demonstrate effectiveness of employing in-lake activities before or with watershed management activities when internal loading dominates in summer 1 Welch and Jacoby, LRM, Cooke et al., Restoration/Management /Lakes/Reservoirs, Dugopolski, Lake Line, 2010 Lake Internal TP % of Total 1 Initial Internal TP % Reduction 2 Summer Lake TP % Reduction 2 Longevity Years 2 Long- Kitsap ( 80) (34) 4 (11) Second ( 91) 54 >4 Third ( 07) 68 >4 Erie ( 85) >8 Campbell ( 85) Long- Thurston ( 83) Pattison ( 83) Green ( 91) Second Green ( 04) > > > >8 10

11 70 60 TP ChL Transparency TP and ChL (µg/l) alum (fall) 1991 alum (fall) 2007 alum (spring) Transparency (m) Years Mean whole-lake summer (June-September) TP, chl and Secchi transparency for groups of study years before and after three alum treatments in Long Lake, Kitsap 11

12 Decrease in internal P loading => decreases in-lake TP and chl even with spikes in external P loading 12

13 Hydraulic Residence Time Affects Lake Recovery Water residence time determines how quickly water column TP will return to higher equilibrium with continued external TP loading after in-lake treatment i.e. Six lake s average residence time = 0.53 yr. (1.89/yr flushing rate) Lake TP remained reduced for 5-8 years after internal P loading reduced Demonstrates the dominance of internal loading largely controlling summer lake TP and chl In contrast, lake response to external P loading reduction: water column TP will recover to new reduced equilibrium in a much longer time GLSM ~40 years Assuming 80% reduction in watershed P loading In most case 80% less is still more than background P loading 13

14 Phosphorus (µg/g dry weight) Sediment Depth (centimeters) Labile-P Fe-P Al-P Organic-P Ca-P Total P 40 P fractions after 2004 treatment of Green Lake (Dugopolski, 2005) Formed Al-P is stable: Al added: Al-P formation in sediments continues to decrease over time until Al added: Al-P formed ended up at ~ 11:1 in 8 WA lakes (Rydin et al.,

15 In-lake Treatments are NOT One Time Activity Just Like Watershed BMPs Al-P and floc layer settles at ~1.5 cm/yr, is mixed by bioturbation and is gradually covered with new sediment (Cooke, et al., 2005) Additional treatments probably necessary: Long Lake-Kitsap: 1980, 1991, 2007 Green Lake: 1991, 2004, 201? After 2004 alum treatment Green Lake experience first HAB in summer of

16 GLSM 5,000 ha area Example: Grand Lake St. Marys 1.6 m mean depth 16 km wind fetch Treated in June 2011 and April 2012 Only 1,960 ha area (40%) both years Treated area at 45 g/m 2 Al in 2011 and 50 g/m 2 Al in 2012; both less than half recommended dose to inactivate sediments Combined doses in 2011 and 2012 equaled 70% of the recommended treatment for the mid-lake area External load huge: Inflow TP = 271 mg/l However, Internal load contributed 91% of Summer TP Load in

17 Internal loading reduced by 37% in 2011, compared to model calibrated rate in

18 Alum Treatment Results Summer mean TP decreased 26% in 2011: 187 to 138 µg/l No decrease in chl or transparency observed in 2011 However less than 37 % of dose used and less than 40 % of sediment area treated Internal loading did decrease from 4.0 mg/m 2 per day in 2010 before treatment to 1.8 mg/m 2 per day in 2012 after two partial treatments $3.5 mil to reduce internal P loading by 37% versus $100 to $200 mil for same % reduction in external P with limited impact on primary production 18

19 Alum Treatment Results TP mass balance modeling showed internal loading decreased from 4.0 mg/m 2 per day in 2010 to 1.8 mg/m 2 per day in 2012 In 2012; relatively low TP concentrations during April and May (during and right after treatment) Initial chl decline after 2012 treatment GLSM had 23% less volume and corresponding mean depth in summer 2012 due to lake operations and drought; greater resuspension of P and internal loading; masked treatment effect Cyanobacteria still extremely high Despite no apparent lasting effect on summer mean TP in the water column, sediment Al and Al-P concentrations showed that the two alum treatments inactivated sediment P as was intended Al to Al bond P increase at the designed 11:1 ratio within the surface sediments of the treated area demonstrating that the treatment was directly impacting internal P availability 19

20 GLSM Sediment Al-P Concentration 20

21 Cost of Watershed Treatment vs In-lake GLSM ag, suburban, urban $20 to $30 mil vs $200 to $300 mil Immediate benefits vs 40 years after 80% reduction (if possible) Long Lake - suburban, urban assume external P loading reduction of 50 % wetlands/bmps $10 to $20 mil In-lake 3 treatment over 22 years total cost $500K Green Lake Urban City Seattle Two alum treatments 20 years benefit $1.1 mil BPMs estimate for P reduction of 50%, $0.9 to $1.4 billion Estimated needed reduction to induce in-lake response 75 to 85% reduction in P loading The 14 PS lakes had average TP inflow of 77 ± 48 µg/l - urban suburban watershed GLSM ~280 µg/l agricultural watershed 21

22 Summary Internal P loading in shallow lakes may be more important than external P loading in summer algal bloom production In shallow lakes even modest flux rates from sediments result in high water column concentrations due to shallowness that may lead to HAB Watershed BMPs will only address part of the increase in external P loading due to land-use compared to historical P loading Alum proven effective in shallow lakes, regardless of the level of watershed management, in reducing internal P loading and HABs Alum is also effective in deep stratified lake where hypolimnetic P becomes available to drive Cyanobacteria blooms 22